1. Field of the Invention:
The present invention relates to a field decision circuit used in a circuit for processing video information, when one frame of video information includes a plurality of fields.
2. Description of the Related Art:
The standards such as NTSC (National Television System Committee) and PAL (Phase Alternation Line) relate to a video signal, when one frame of the video signal includes a plurality of fields. In order to process the video signal according to the standards, a field judgment circuit may be required for specifying which field is currently processed of the plurality of fields. However, it is difficult to completely eliminate any incorrect operation of the field judgment circuit because of noise, variation in the timing of the synchronizing signal, or the like. Further, in driving a flat-type display apparatus which has been recently developed, more particularly, a liquid crystal display device, it is extremely important to correctly judge the field. This is because the judgment result of the field is used as information for determining the position of video information which is to be reproduced on a display panel, and is also indispensable as information for alternating the polarity of voltages applied to pixels.
Japanese Laid-Open Patent Publication No. 4-138775 discloses a conventional field decision circuit for providing correct field information by modifying the incorrect judgment result from the field judgment circuit, when the field judgment circuit operates incorrectly.
FIG. 4 shows a configuration of the conventional field decision circuit. The field decision circuit includes a first memory 41 for storing the output of the field judgment circuit over a predetermined number of fields, a second memory 42 for storing the output of the field decision circuit over the predetermined number of fields, and a comparator 43 for comparing the contents of the first memory 41 with the contents of the second memory 42.
When only the field decision circuit or only the field decision circuit in combination with the field judgment circuit is implemented as an LSI (Large Scale Integration), the number of gates required for the whole circuit(s) is not large. However, when the field decision circuit is used as a part of a video signal processing unit and is implemented in an LSI for the video signal processing unit, the number of gates required for the whole circuit(s) is relatively large. As a result, the number of gates of the field decision circuit possibly influence the cost of the LSI for the video signal processing unit. The influence becomes noticeable when a gate array is used as the LSI for the video signal processing unit.
For example, it is assumed that a gate array having 1,000 gates is used as the LSI for the video signal processing unit. In this case, the number of gates which can be actually used is at most 900 because of restrictions concerning the arrangement of elements and interconnections between elements. In a gate array, since it is common that almost all gates which can be actually used are already used by the existing circuits, it is often difficult to add another circuit requiring many gates to the gate array. Accordingly, in a case where the field decision circuit requires many gates, the number of the required gates possibly exceeds 1,000, which is the maximum allowable number of gates in the gate array.
In a case where the number of required gates exceed 1,000, it is necessary to take additional measures. One measure is to decrease the number of fields to be stored in the field decision circuit so as to decrease the number of gates in the gate array. This makes it possible to decrease the number of memory elements such as flip-flops. Since the memory elements require a relatively large number of gates, the decrease of the number of memory elements causes a great effect to the decrease of the number of gates in the gate array. However, the measures have a drawback in that the reliability of the field decision circuit is degraded. Another measure is to use a ranked-up gate array, for example, a gate array having 2,000 gates, as the LSI for the video signal processing unit. However, this measure has a drawback in that the cost of the LSI becomes high.